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Block sliding down frictionless ramp while being pulled with constant force?

  1. Oct 3, 2007 #1
    1. The problem statement, all variables and given/known data
    A block of ice slides down a frictionless ramp at theta = 60 degrees while an ice worker pulls on the block (via a rope) with a constant force of 50.0 N. The block slides a distance d = 0.8m along the ramp and its kinetic energy increases by 128 J.
    (a) Calculate the work done on the block by the gravitational force.
    (b) Calculate the work done on the block by the normal force.
    (c) Calculate the percent increase in its kinetic energy if it slid down the ramp the same distance with no rope attached.


    2. Relevant equations
    I have no idea, that's my problem!


    3. The attempt at a solution
    I don't even know where to start!
     
  2. jcsd
  3. Oct 3, 2007 #2
    Do you know the formulas for work and concept of free body diagram(FBD, in short) ?
     
    Last edited: Oct 3, 2007
  4. Oct 3, 2007 #3
    no and yes...

    I understand the concept of free body diagrams, but there are so many formulas that I really need help choosing which ones to use for this specific problem.
     
  5. Oct 3, 2007 #4
    OK. Draw a free body diagram.
    For I and II : W = F.r
    You know F from FBD for both cases and r is given to you. Use your "intelligence" to find the angle between F and r and calculate the dot product in each case.
     
    Last edited: Oct 3, 2007
  6. Oct 3, 2007 #5
    This may be a really stupid question, but what does "r" stand for? Is it displacement?
     
  7. Oct 3, 2007 #6
    r really should be s or distance i think...
     
  8. Oct 3, 2007 #7
    If it has a constant force then that must mean that F=ma is constant, therefore its acceleration isnt changing, as this would alter the force, so how is the block of ice gaining Ek due to a change in velocity?
     
  9. Oct 3, 2007 #8
    yeah, I don't get it either! and don't I need to know the mass of the block of ice?? I'm so confused...
     
  10. Oct 3, 2007 #9

    learningphysics

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    Homework Helper

    If there is an acceleration, then velocity is changing.
     
  11. Oct 3, 2007 #10

    learningphysics

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    you don't need the mass.

    Net work done by all forces = change in kinetic energy.

    There are 3 forces to consider here. Gravity, the normal force, and the 50N force.

    What is the work done by the normal force?

    What is the work done by the 50N force?

    [tex]W_{gravity} + W_{normal} + W_{50} = 128[/tex]

    you can solve for Wgravity using this equation. so the idea is to solve part b) before part a).
     
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